Generally, the paper manufacturing process employs a machine that systematically de-waters pulp slurry. The pulp slurry consists largely of cellulose wood fibers, along with various chemical additives used as fillers and functional components of the paper or paper products. The pulp is prepared from various species of wood. Two pulping methods include chemical digestion to separate the cellulose fibers from lignin and other natural organic binders, or mechanical grinding and refining. The resulting cellulose fibers are used in the manufacture of paper products, whereby the pulp is supplied to a paper machine system, slurried in water to various solids levels (termed “consistency”), and ultimately diluted to about 0.5-1.0% solids for subsequent de-watering to form a sheet of paper. This low consistency of solids of the pulp facilitates fast drainage on the paper former, while also achieving proper fiber-to-fiber contact and orientation in the sheet. De-watering begins on the paper former, which is a synthetic wire or mesh that permits drainage to form a wet-web.
The wet-web is then transferred into the machine press section and is squeezed between roller nips and synthetic press felts (predominantly comprised of nylon) to further remove water. The web is further transferred through a dryer section comprised of steam-heated roller cans. Finally, the sheet is wound onto a reel. Other process stages can include on-machine surface sizing, coating, and/or calendaring to impart functional paper characteristics.
Generally, the wet-web is approximately 20% solids coming off the former, 40% solids after leaving the press section, and about 94-97% solids (3-6% moisture) as the paper on the reel. Various chemical compounds are added to the fiber slurry to impart certain functional properties to different types of paper. Fillers such as clay, talc, titanium dioxide, and calcium carbonate may be added to the slurry to impart opacity, improve brightness, improve sheet printing, substitute for more expensive fiber, improve sheet smoothness, and improve overall paper quality. Additionally, various organic compounds are added to the fiber slurry to further enhance paper characteristics. These organic compounds include, but are not limited to: starch for internal fiber bonding strength, retention aids to help hold or bind the inorganic fillers and cellulose fines in the sheet; brightening compounds; dyes; sizing agents to improve sheet printing so that the ink doesn't bleed through the sheet; as well as various other organic compounds. Sizing agents may include acid rosin, alkaline alkyl-ketene-dimer (AKD), and alkaline alkenyl-succinic-anhydride (ASA). Therefore, as the sheet is de-watered on the paper machine, many types of deposits may result on the papermaking equipment. These deposits can result from the chemicals used in the process, natural wood compounds that are not thoroughly removed from pulping processes, or from inclusion of recycled fiber in the pulp slurry as a result of water re-use.
The primary function of the press felts, other than a means of sheet conveyance, is to aid in the de-watering process of the wet-web. The press felts absorb, receive, and transport water that is expressed from the wet-web by the pressure of the roller nips. On most modern paper machines, the water is subsequently removed from the press felts by vacuum elements in the press, the vacuum elements consisting of the Uhle boxes and suction press rolls. The press felts then return in their travel loop back to the nip, and continually receive and transport water away from the web. Consequentially, the press felts become contaminated with various types of soils resulting from both the web compounds and from the process shower waters used to flush the press felts.
One type of soil found in a wide variety of papermaking processes are hydrogels. These soils are found in papermaking felts including press felts, through air drying (TAD) woven belts, forming wires and other dewatering elements found on papermaking and pulp machines. These hydrogels typically consists of polymeric materials including but not limited to wet strength resins, dry strength resins, retention aids, drainage aids, dry strength additives, Yankee coating additives and microbial derived slime. These additives include but are not limited to those derived from: starches, modified starches (cationically, anionically and hydrophobically modified), polyaminoamido epichlorohydrin (PAE) derived polymers, glyoxylated polyacrylamide resins, polyvinyl alcohols, polyacrylamide polymers, polyacrylamide/polyvinyl acetate copolymers, polyvinyl/polyvinyl acetate copolymers, and cationic derivatives of the aforementioned. In addition, hydrogels may include carboxyl methylcellulose, guar gum, and similarly derived retention aids, including but not limited to polyamines and polydiallyldimethylammonium chloride (PDADMAC).
Generally speaking, hydrogels consist of polymers which absorb water in the range of solids to water ratio of 1:2 to 1:200, and typically in the ratio of 1:10 to 1:100. Another name for hydrogels are wet soils. Another name for hydrogels is putty or wet putty. Hydrogels may also be described simply as gels.
Another type of hydrogels may be derived from the activity of microbial organisms in a press felt, which may secrete a gelatinous material commonly known as bacterial or microbial slime. Generally, these may consist of polysaccharides, glycoprotiens, or a combination of these materials.
These hydrogels will occupy a void volume disproportionate to their weight due to absorbed water, and thereby rendering the felt less able to absorb water from the wet web of paper in press section of a paper machine. As a consequence, the press felt is rendered less capable of performing its function of removing water from the paper web. This greatly reduces the performance of a paper machine in making a dry sheet of paper. Furthermore this may cause the papermachine to run more slowly, because defects in the paper, reduce paper machine output, increase the consumption of other papermaking chemicals, decrease the useful life of the papermaking felts, and generally cause papermachine instability.
Hydrogels are difficult to remove from press felts using conventional cleaners for a number of reasons. Due to their polymeric structure they are especially resistant to the action of acids, solvents, and cleaning agents. Alkaline cleaners may cause hydrogels to absorb even more water, further decreasing the ability of press felts to absorb and remove water from the paper web. The polymer bonds tend to be resistant to chemical breakdown by conventional cleaners.
Sometimes hydrogels may be removed from press felts by shutting down a paper machine and applying high levels of alkaline cleaners such as a sodium or potassium hydroxide and or oxidizing agents such a sodium hypochlorite or potassium persulfate. This strategy necessitates that the paper machine be taken out of production. This is undesirable, since the resulting loss of production incurs a large expense to the paper manufacturer.
As such, there is a decrease in the performance of the felts and interference with the efficient operation of the paper machine, often resulting in: speed reductions, sheet crushing, quality defects, excess energy consumption, holes and possibly machine downtime and increased costs.